1. Field of the Invention:
The present invention relates to a solenoid valve driving device and, in particular, relates to a driving device for driving a plurality of solenoid valves which are parallel-supplied with electric power by an electric power supply source in common.
2. Related Art:
A solenoid coil of a solenoid valve which has been used for an anti-skid control device and has been operated under duty control (PWM control) has been driven by a driving circuit as shown in FIG. 9. In the driving device shown in FIG. 9, a first solenoid valve 3 is operated under duty control. In duty control, the mean current flowing through the first solenoid valve 3 is controlled by switching a first driving IC 4.
The driving circuit shown in FIG. 9 is provided with the first and the second driving ICs 4, 6 which receive and are driven by respective driving signals from a central processing unit (hereinafter referred to as "CPU 7"). The CPU 7 sends the electronic signals to the first and the second driving ICs 4, 6 via a buffer not shown or the like. When the driving ICs 4, 6 are turned on, a current from an electric power supply source 1 is supplied to the first solenoid valve 3 and the second solenoid valve 5 corresponding to the first driving IC 4 and the second driving IC 6, respectively. For example, when the first driving IC 4 is turned on, current flows from the electric power supply source 1 to the first solenoid valve 3 and the first driving IC 4 via a diode 2 and a choke coil 9. Immediately after the first driving IC 4 is turned off, the recirculation of the current occurs through the first solenoid valve 3 and a recirculation diode 8 which is connected in parallel to the first solenoid valve 3. A recirculation diode may be connected in parallel to the second solenoid valve 5, however, since it produces the same effect as the recirculation diode 8, the representation in the figure and the description thereof will be omitted. If the second solenoid valve 5 is not operated under duty control, the recirculation diode is not required for the second solenoid valve 5.
In this circuit structure, there is a problem that, when the first driving IC 4 is switched from an OFF-state (shut-down state) to an ON-state (conductive state) while the first solenoid valve 3 is operated under duty control, the recirculation diode 8 connected in parallel to the first solenoid valve 3 does not come to a state in which it prevents the current flowing in a reverse direction at the same time when the first driving IC 4 is turned on. That is, the recirculation diode 8 comes to a reverse direction current prevention state (a one- way current passing state) in which current is prevented from flowing from the electric power supply source 1 to the first driving IC 4 through the recirculation diode after an instantaneous delay time has passed since the first driving IC 4 is turned on. During this delay time, bypassing current flows from the electric power supply source 1 to the first driving IC 4 through the recirculation diode 8 (hereinafter referred to as "through current"). Since the change of the through current is very rapid, surge voltage is produced by the parasitic inductance of the wire in the circuit. As a result, a problem is produced such that the recirculation diode 8 and the first driving IC 4 are required to have a large margin on withstand voltage. In addition, the surge voltage and the surge current cause radio-noises.
Therefore, as shown in FIG. 9, a choke coil 9 has been provided in the upstream of the first solenoid valve 3 as a coil for preventing a rapid change of the through current. However, the choke coil provided in the circuit increases the number of parts and a production cost. In particular, if there are many solenoid valves driven by a solenoid valve driving device, for example, in the case of an anti-skid control device, many choke coils are required to prevent the surge voltage with the result that the production cost is further increased. If the choke coil 9 is not used, high-response recirculation diodes having a small delay in response need to be used, which also increases the production cost.